Radio access technologies for cellular mobile networks are continuously being evolved to meet future demands for higher data rates, improved coverage, and capacity. One example is the evolution of the WCDMA access technology to provide High-Speed Packet Access (HSPA). With such evolution to higher data rates, the power contributions of users in neighboring cells, which is called inter-cell interference, becomes more significant. FIG. 1 illustrates an example of a mobile radio (shown as a laptop computer) near a border between cell A and cell B. Base station A serves the mobile radio, and base station B is a non-serving base station relative to the mobile radio. As depicted with two arrows, the uplink transmission from the mobile radio is received at both base stations at about the same signal strength. In cell A, that uplink transmission is a desired signal, but in cell B, it is inter-cell interference that adversely impacts the communications quality, capacity, and throughput in cell B. To maintain communications quality, capacity, and throughput in neighboring cells, efficient and effective inter-cell interference control is needed. Inter-cell interference control is also useful for admission and congestion control as well as resource control and allocation, all of which are generally referred to as resource management.
The total received wideband power at a base station includes background noise power in the base station and the sum of the received power from all transmitting mobiles in serving and non-serving cells. The noise rise is the ratio of the total received wideband power to the background noise power. In the uplink, the common resource shared among the mobile radio terminals is the amount of tolerable interference, i.e., the total received power, or the noise rise at the base station. FIG. 2(a) is a graph of the noise rise in serving and neighboring cells that illustrates the inter-cell interference contributed by a mobile radio near a cell border. The amount of power that each mobile radio contributes to the total received wideband power depends on the data rate and the radio path gain associated with the mobile radio. Hence, the received power from a mobile radio is the uplink transmit power multiplied with the path gain (PG) of the radio link. FIG. 2(b) graphs the received power in terms of path gain (PG) of the uplink transmission from the mobile radio to the serving and neighboring cell. Just before a new soft handover (SHO) link is established for the mobile connection, the noise rise and power contributions (PG) of the mobile radio increase dramatically (as the mobile gets closer to the adjacent cell) and then decrease when the soft handover (SHO) link is established with the neighboring base station. In this illustration, the maximum data rate for the soft handover example was limited, e.g., to 128 kbps, using a scheduling grant issued by the neighbor base station to the mobile radio as part of the SHO operation, which is one way to limit the inter-cell interference from mobile radios near a cell border.
The more mobile radios transmitting, the more interference, and the higher the uplink load is in that base station's cell. Unfortunately, it is difficult to determine for a neighboring cell the inter-cell interference impact that an uplink mobile transmission from a mobile radio will have that is not served by the base station in that cell. Determining the impact that the mobile's transmission will have on another cell is particularly problematic in decentralized or distributed resource management schemes. Distributed resource control is desirable because it is implemented much “closer” to where the resources are actually used. Given the trend towards high speed downlink and uplink transmission formats, resource management is more decentralized or distributed in order to achieve higher speeds and avoid the considerable signaling (and associated costs) required for centralized control.
Although a centralized resource manager receives information from various cells, which allows informing base stations about mobile connections, conditions, etc., in adjacent cells, a distributed resource manager in a base station, e.g., a scheduler, typically does not have information about other mobile connections it is not supervising/serving. Assume that a high power or high data rate uplink transmission from a mobile station served by a serving base station in a first cell creates significant interference in a nearby non-serving cell managed by a second base station. That interference increases the load in the non-serving cell and effectively consumes resources in the non-serving cell that the non-serving base station would rather use to service mobiles actually within its cell. The non-serving base station itself has no way of directly knowing the inter-cell interference impact that other mobile uplink transmissions will have on its resources or how the inter-cell interference will impact current communications being supported in the non-serving cell. Nor does the serving base station know the contribution its served mobiles' transmissions make to the interference in the non-serving cell.
Scheduling may be used to determine when a certain mobile terminal is allowed to transmit and at what maximum data rate. With scheduling, the base station may influence is the mobile terminal's transport format (e.g., TFC) selection for the uplink transmission, e.g., over an enhanced uplink channel like the E-DCH. Two types of grants may be used: absolute grants and relative grants. Absolute grants set an absolute value of the upper limit of the power the user terminal may use for data transmission. The maximum power allowed for a data transmission determines the maximum data rate for the uplink communication. Relative grants update the resource allocation for a terminal and may assume one of three values: up, down, or hold, to instruct the terminal to increase, decrease, or not change uplink transmitting power based on the amount of radio resources the terminal is currently using. An absolute grant to a user terminal is usually only sent by the cell serving the user terminal, while the relative grants can be transmitted from both serving and non-serving cells.
Because only the mobile radio terminal knows the buffer and power situation at the time of its uplink transmission, (i.e., how much data the mobile needs to be sent and the power needed to do so in a particular time period), the base station scheduler in the serving cell can only send a maximum uplink power limit, e.g., in a scheduling grant, above which the mobile is not allowed to transmit. That limit may be expressed as a specific transport format (e.g., a particular TFC) or as a maximum data rate.
There are problems with trying to control inter-cell interference using a relative grant. First, a relative grant from a non-serving cell is possible only when the mobile radio is in a soft handover, as in the example shown in FIGS. 2(a) and 2(b), because a mobile can only receive a relative grant sent from cells in its active cell set. As a result, a neighbor cell cannot control the interference from mobile radios that are not in soft handover. Second, even if soft handover connections are possible with neighbor cells, it takes time and radio resource control (RRC) signaling to add soft handover links, which should be avoided unless soft handover is warranted for reasons other than inter-cell interference monitoring and control.
What is needed is a way to estimate the inter-cell interference caused by uplink transmissions to the neighboring cells, detect when the estimated inter-cell interference exceeds a threshold, and manage the radio resources in a serving cell to reduce the estimated inter-cell interference so that it is less than the threshold.